It would be really useful to have an upmix plugin, the specific use case I am thinking of is for bus sends, ideally I'd like to send the same amount of signal to all surround channels then mute unwanted and mix within the splitter tool, or via the plugin wrench.
Mixtool seems like an ideal candidate for this, it seems it already works like an upmixer on surround channels.
Perhaps if Mixtool was able to be used instead of the panner on sends as an option ? That would be ideal.